The present invention relates to the art of industrial controllers, and more particularly to a control system and methodology for controlling pump cavitation and blockage.
Motorized pumps are employed in industry for controlling fluid flowing in a pipe, fluid level in a tank or container, or in other applications, wherein the pump receives fluid via an intake and provides fluid to an outlet at a different (e.g., higher) pressure and/or flow rate. Such pumps may thus be employed to provide outlet fluid at a desired pressure (e.g., pounds per square inch or PSI), flow rate (e.g., gallons per minute or GPM), or according to some other desired parameter associated with the performance of a system in which the pump is employed. For example, the pump may be operatively associated with a pump control system implemented via a programmable logic controller (PLC) coupled to a motor drive, which controls the pump motor speed in order to achieve a desired outlet fluid flow rate, and which includes I/O circuitry such as analog to digital (A/D) converters for interfacing with sensors and outputs for interfacing with actuators associated with the controlled pump system. In such a configuration, the control algorithm in the PLC may receive process variable signals from one or more sensors associated with the pump, such as a flow meter in the outlet fluid stream, and may make appropriate adjustments in the pump motor speed such that the desired flow rate is realized.
In conventional motorized pump control systems, the motor speed is related to the measured process variable by a control scheme or algorithm, for example, where the measured flow rate is compared with the desired flow rate (e.g., setpoint). If the measured flow rate is less than the desired or setpoint flow rate, the PLC may determine a new speed and send this new speed setpoint to the drive in the form of an analog or digital signal. The drive may then increase the motor speed to the new speed setpoint, whereby the flow rate is increased. Similarly, if the measured flow rate exceeds the desired flow rate, the motor speed may be decreased. Control logic within the control system may perform the comparison of the desired process value (e.g., flow rate setpoint) with the measured flow rate value (e.g., obtained from a flow sensor signal and converted to a digital value via a typical A/D converter), and provide a control output value, such as a desired motor speed signal, to the motor drive according to the comparison.
The control output value in this regard, may be determined according to a control algorithm, such as a proportional, integral, derivative (PID) algorithm, which provides for stable control of the pump in a given process. The motor drive thereafter provides appropriate electrical power, for example, three phase AC motor currents, to the pump motor in order to achieve the desired motor speed to effectuate the desired flow rate in the controlled process. Load fluctuations or power fluctuations which may cause the motor speed to drift from the desired, target speed are accommodated by logic internal to the drive. The motor speed is maintained in this speed-control manner based on drive logic and sensed or computed motor speed.
Motorized pumps, however, are sometimes subjected to process disturbances, which disrupt the closed loop performance of the system. In addition, one or more components of the process may fail or become temporarily inoperative, such as when partial or complete blockage of an inlet or outlet pipe occurs, when a pipe breaks, when a coupling fails, or when a valve upstream of the pump fluid inlet or downstream of the pump discharge fluid outlet becomes frozen in a closed position. In certain cases, the form and/or nature of such disturbances or failures may prevent the motorized pump from achieving the desired process performance. For instance, where the pump cannot supply enough pressure to realize the desired outlet fluid flow rate, the control system may increase the pump motor speed to its maximum value. Where the inability of the pump to achieve such pressure is due to inadequate inlet fluid supply, partially or fully blocked outlet passage, or some other condition, the excessive speed of the pump motor may cause damage to the pump, the motor, or other system components.
Some typical process disturbance conditions associated with motorized pumps include pump cavitation, partial or complete blockage of the inlet or outlet, and impeller wear or damage. Cavitation is the formation of vapor bubbles in the inlet flow regime or the suction zone of the pump, which can cause accelerated wear, and mechanical damage to pump seals, bearing and other pump components, mechanical couplings, gear trains, and motor components. This condition occurs when local pressure drops to below the vapor pressure of the liquid being pumped. These vapor bubbles collapse or implode when they enter a higher-pressure zone (e.g., at the discharge section or a higher pressure area near the impeller) of the pump, causing erosion of impeller casings as well as accelerated wear or damage to other pump components.
If a pump runs for an extended period under cavitation conditions, permanent damage may occur to the pump structure and accelerated wear and deterioration of pump internal surfaces, bearings, and seals may occur. If left unchecked, this deterioration can result in pump failure, leakage of flammable or toxic fluids, or destruction of other machines or processes for example. These conditions may represent an environmental hazard and a risk to humans in the area. Thus, it is desirable to provide improved controllers and pump control systems for motorized pumps, which minimize or reduce the damage or wear associated with pump cavitation and other process disturbances, failures, and/or faults associated with motorized pumps and pumping processes.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Rather, the sole purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented hereinafter. The invention provides control apparatus and methods by which damage and other problems associated with motorized pump cavitation or other faults may be minimized or eliminated, and which provide for specialized control of pumps during actual or suspected cavitation conditions or other process upset conditions, and for possible resumption of normal control once an actual or suspected cavitation condition ends.
One aspect of the invention provides a pump control system for operating a motorized pump in a controlled fashion. The control system comprises a motor drive providing electrical power to operate the pump motor according to a motor control signal, and a controller with a cavitation detection component operatively connected to the pump system (e.g., which may include sensors on the pump or connecting pipes) to detect actual or suspected cavitation in the pump. The controller provides the control signal to the motor drive which represents a determined desired motor speed value. The desired motor speed, in turn, is based on a pump system setpoint or a cavitation signal determined from the cavitation detection component according to control logic for avoiding or preventing cavitation in the pump. In this manner, the controller accounts for actual, likely, or incipient cavitation conditions in providing the motor control signal.
For instance, where cavitation is suspected, the controller may provide the control signal according to the cavitation signal, which may operate to slow the motor down to avoid actual cavitation and/or until an actual cavitation condition subsides, whereby damage or premature wear heretofore experienced as a result of pump cavitation may be avoided or reduced. The controller may maintain such pump operation until the cavitation condition (e.g., actual or suspected) ends, whereafter normal control is resumed, or alternatively the controller may operate the pump near the cavitation condition to prevent unstable operation. The cavitation detection component of the system may detect actual, likely, or incipient cavitation through one or more computations based on measured process values and stored process and equipment data. For example, cavitation may be suspected when net positive suction required exceeds or is near net positive suction available in the pump.
Another aspect of the invention provides a controller for providing a control signal to a motor drive to operate a motorized pump in a controlled fashion. The controller may comprise a cavitation detection component adapted to detect cavitation in the pump, wherein the controller provides the control signal to the motor drive according to a setpoint or a cavitation signal from the cavitation detection component according to detected cavitation in the pump.
Still another aspect of the invention provides a method of controlling a motorized pump, by which damage or other deleterious effects previously associated with cavitation in a motorized pump are alleviated. The method may comprise detecting cavitation in the pump, controlling the pump according to a process setpoint if no cavitation is detected in the pump, and controlling the pump according to a cavitation signal if cavitation is detected in the pump. The method may be advantageously employed in order to reduce or eliminate the previously encountered problems associated with motorized pump cavitation.
Yet another aspect of the invention provides a method to control a pump according to a process setpoint if sufficiently far from any possible cavitation condition. If a cavitation detection signal indicates no cavitation, but the process is sufficiently near possible cavitation, then the method may be advantageously employed to maintain a suitable margin away from possible cavitation.
Another aspect of the invention provides a method to control a pump according to a process setpoint if sufficiently far from any possible cavitation condition. If cavitation is detected to a slight degree, the method may be advantageously employed to limit the degree of cavitation and to prevent severe cavitation, but to permit the process to continue with a controlled minimum amount of cavitation. Such conditions may be required, for example, when emptying a tank car and insufficient head is available for the minimum flow required to empty the tank without cavitation.
Yet another aspect of the invention provides a method to control a pump according to a process setpoint if sufficiently far from any possible cavitation condition. If cavitation is detected to a slight degree, the method may be advantageously employed to increase the degree of cavitation slightly to prevent severe cavitation but also to avoid a minimum cavitation condition which may be more damaging to equipment then a slightly greater degree of cavitation. This will permit the process to continue with a controlled target amount of cavitation and controlled machinery damage and wear.